ipc_service: Automatically include source files for backends

The purpose of this change is to allow to enable more than one
backend at once by removing choice from ipc-service backend Kconfig
and depending backend Kconfig option on existing of correct compatible.

Overwriting IPC_SERVICE_BACKEND option in some places is removes
as no longer needed.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>